Exploiting ghost-FSMs as a BIST structure for sequential machines

This paper introduces the novel concept of ghost-FSM as a BIST structure for sequential machines. A ghost-FSM is a contrived implicit machine that is deliberately embedded within the framework of a given FSM. A ghost-FSM remains dormant except at the testing phase, when it helps to generate those test patterns that, otherwise, could not be generated due to certain idiosyncrasies of finite state machines. It acts as an effective tool to enhance the fault coverage of a sequential machine in a PRPG based BIST environment, without affecting the normal operating mode of the machine. Extensive experimentation, carried out on MCNC benchmarks, confirm that significant improvement in testability is attained with occasional, very little, area overhead.